Drop use of get_os_version_codename_swift

Swift payload upgrades to wallaby or later currently fail because
this code is unable to determine the version for wallaby+. For
example:

FATAL ERROR: Could not derive swift version for codename: wallaby

We stopped maintaining the SWIFT_CODENAMES and PACKAGE_CODENAMES as of
wallaby because the openstack-release package was introduced in wallaby.

We could update the SWIFT_CODENAMES map, but really this map no longer
needs to be used. There was a time when the same swift package version
existed in two different OpenStack releases, but that is no longer the
case [1]. Therefore we can use the same comparison used for OpenStack
packages which essentially checks current release (based on the
openstack-release package) vs the installation source release.

[1] Current swift versions in Ubuntu are:
2.32.0 bobcat
2.31.1 antelope
2.30.1 zed
2.29.2 yoga
2.28.1 xena
2.27.0 wallaby
2.26.0 victoria
2.25.2 ussuri

Closes-Bug: #2040606

charmhelpers/contrib/openstack/utils.py

@@ -846,14 +846,10 @@ def openstack_upgrade_available(package):
     if not cur_vers:
         # The package has not been installed yet do not attempt upgrade
         return False
-    if "swift" in package:
-        codename = get_os_codename_install_source(src)
-        avail_vers = get_os_version_codename_swift(codename)
-    else:
-        try:
-            avail_vers = get_os_version_install_source(src)
-        except Exception:
-            avail_vers = cur_vers
+    try:
+        avail_vers = get_os_version_install_source(src)
+    except Exception:
+        avail_vers = cur_vers
     apt.init()
     return apt.version_compare(avail_vers, cur_vers) >= 1